1. Field of the Invention
The present invention relates to antennas for electromagnetic waves and, more particularly, to RF antennas whose radiation pattern may be scanned electronically.
2. Description of the Prior Art
In many fields of electronics, it is often necessary to scan the radiation pattern of antennas.
Ferroelectric materials have a number of attractive properties. Ferroelectrics can handle high peak power. The average power handling capacity is governed by the dielectric loss of the material. They have low switching time (such as 100 nS). Some ferroelectrics have low losses. The permittivity of ferroelectrics is generally large, as such the device is small in size. The ferroelectrics are operated at a constant temperature in the paraelectric phase i.e. slightly above the Curie temperature. The scanning part of the ferroelectric scanning RF antenna can be made of thin films, and can be integrated with other monolithic microwave/RF devices. Inherently, they have a broad bandwidth. They have no low frequency limitation as in the case of ferrite devices. The high frequency operation is governed by the relaxation frequency, such as 95 GHz for strontium titanate, of the ferroelectric material. The loss of the ferroelectric scanning RF antenna is low with ferroelectric materials with a low loss tangent. A number of ferroelectric materials are not subject to burnout. The ferroelectric scanning RF antenna is a reciprocal device i.e. it can be used for transmission and reception.
The optical deflection and modulation by a ferroelectric device has been studied. F. S. Chen, J. E. Geusic, S. K. Kurtz, J. G. Skinner and S. H. Wemple, "Light Modulation and Beam Deflection with Potassium-Tantalate-Niobate Crystals," J. Appl. Phys. vol. 37, No.1, pp. 388-398, January 1966 and T. Utsunomiya, K. Nagata and K. Okazaki, "Prism-Type Optical Deflector Using PLZT Ceramics," Jap. J. Appl. Phys. vol.24, Suppl. 24-3, pp. 169-171, 1985. A liquid ferroelectric optical switch has been reported. S. S. Bawa, A. M.Bindar, K. Saxena and Subhas Chandra, "Miniaturized total reflection ferroelectric liquid-crystal electro-optic switch," App. Phys. Lett. 57 (15), pp. 1479-81, 8 Oct. 1990.
In the U.S. Pat. No. 5,304,960 Das claimed ferroelectric total internal reflection switch. An antenna was fabricated by cutting periodic grooves into the side wall of an optimized ferrite-type dielectric waveguide, thereby forming a series of radiating elements. R. A. Stern, R. W. Babbitt and J. Borowick, A mm-wave Homogeneous Ferrite Scan Antenna," Microwave Journal, pp. 101-108, April 1987.
Ferroelectric scanning apertures have been discussed by Das. S. Das, "Scanning Ferroelectric Apertures," The Radio and Electronic Engineer, pp. 263-268, May 1974.
However, the impedance of the ferroelectric scanning aperture is very low and the efficiency of its radiation is very small. The present invention presents a high efficiency ferroelectric scanning RF antenna. The invention also presents (1) a thin film structure of the scanning section of the ferroelectric scanning RF antenna, (2) the use of ferroelectric liquid crystal as the scanning section and (3) the use of high Tc superconductor material in place of silver or gold type conductive material to reduce the conductive loss and thus increase the efficiency of the ferroelectric scanning RF antenna.
There are significant differences between the RF and optical deflectors. In the optical deflector, the light ray travels through a very small portion of the scanning section. In the scanning RF antenna, the RF energy will travel through the entire portion of the scanning section. The wavelength of RF is several orders of magnitudes greater than the optical wavelengths.
The dimensions of the optical deflector are many times the optical wavelengths. The optical beam diameter is many times the optical wavelength. The width of scanning part of the scanning antenna is generally a fraction of the RF wavelength. The biasing circuit, for the optical deflector, is far away from the optical beam. The biasing circuit, in the case of the RF antenna, has to be isolated, by design, from the RF circuit. The biasing field, in the case of the optical deflector, can be parallel or perpendicular to the direction of the electrical field of the optical beam. For the RF antenna the direction of the biasing field is parallel to the direction of the electrical field of the RF beam.